<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style type="text/css">
		#box{
			width:200px;
			height: 200px;
			background: red url() 0 0 no-repeat;
		}
	</style>
	<script type="text/javascript">
		
		window.onload = function (){

			box = document.getElementById('box');

			box.style.width;

			w = window.getComputedStyle ? window.getComputedStyle(box,null).width : box.currentStyle.width; 

			alert(w);


			// $('#box').width();
		}
	</script>

</head>
<body>
	<div id="box"></div>
</body>
</html>